Mercury is unique among terrestrial planets because of its very high metal/silicate ratio. The NASA MESSENGER mission ended in 2015 and significantly advanced our understanding of the deep structure of the planet as well as our comprehension of Mercury’s volcanic history and crust formation. The European Space Agency (ESA) recently launched the BepiColombo mission in October 2018. The “Planet Mercury” issue summarizes the latest developments in light of MESSENGER data and perspectives for the BepiColombo mission on internal structures and surface processes, based on insights from geophysics, geochemistry, igneous and experimental petrology and volcanology.

Volume 15, Number 2 (April) • Reactive Transport Modeling

Reactive transport modeling, or computer simulations of the transfer of mass and energy through the subsurface, has become a central tool for understanding how Earth’s unique chemical environments are formed, how they function today, and how they might behave in the future. This process-based approach has enabled us to gain new understanding of a diverse array of earth processes, from biogeochemical cycles in marine sediments and the factors that control soil formation, to the evolution of contaminated groundwater systems and the engineered containment of nuclear waste. This issue brings together diverse contributions highlighting the unique role that reactive transport models have played in advancing our understanding of Earth’s shallow crustal environments and human interactions with them.

Using Reactive Transport Models for Understanding, Quantifying and Predicting Groundwater Quality Changes Henning Prommer (University of Western Australia, CSIRO, and National Center for Groundwater Research and Training, Australia) Jing Sun (University of Western Australia and CSIRO, Australia) and Benjamin D. Kocar (Massachusetts Institute of Technology, USA)

Thematic Articles

Unique physical and chemical characteristics of Mercury have been revealed by measurements from NASA’s MESSENGER spacecraft. The closest planet to our Sun is made up of a large metallic core that is partially liquid, a thin mantle thought to be formed by solidification of a silicate magma ocean, and a relatively thick secondary crust produced by partial melting of the mantle followed by volcanic eruptions. However, the origin of the large metal/silicate ratio of the bulk planet and the conditions of accretion remain elusive. Metal enrichment may originate from primordial processes in the solar nebula or from a giant impact that stripped most of the silicate portion of a larger planet leaving Mercury as we know it today.

The planet Mercury is sufficiently close to the Sun to pose a major challenge to spacecraft exploration. The Mariner 10 spacecraft flew by Mercury three times in 1974–1975 but viewed less than half of the surface. With the three flybys of Mercury by the MESSENGER spacecraft in 2008–2009 and the insertion of that probe into orbit about Mercury in 2011, our understanding of the innermost planet substantially improved. In its four years of orbital operations, MESSENGER revealed a world more geologically complex and compositionally distinctive, with a more dynamic magnetosphere and more diverse exosphere–surface interactions, than expected. With the launch of the BepiColombo dual-orbiter mission, the scientific understanding of the innermost planet has moved another major step forward.

NASA’s MESSENGER spacecraft orbited Mercury from 2011 to 2015 and has provided new insights into the interior of the innermost planet. Mercury has a large metallic core ~2,000 km in radius covered by a thin layer of rock only ~420 km thick. Furthermore, a surprisingly large fraction of this outer layer was produced by melting of deeper rocks, forming a light crust ~35 km thick. The core is now known to produce a magnetic field that has intriguing similarities and differences compared to Earth’s field. Some rocks near the surface are magnetized, and the strongest magnetizations are likely to be >3.5 billion years old. This new understanding of Mercury’s interior is helping reveal how rocky planets operate.

Mercury’s volcanic nature has been revealed by NASA’s MESSENGER mission. We now know that all, or most, of the surface has, at some point, been flooded by lavas, sometimes in extremely voluminous eruptions. The ages of Mercury’s lava surfaces reveal that large-volume effusive volcanism ceased about 3.5 billion years ago due to planetary cooling. Mercury’s crust then went into a state of global contraction, thereby impeding further magma ascent. However, some smaller-scale volcanism continued at zones of crustal weakness, particularly at impact craters. Much of this later volcanism has been violently explosive, with volatile gases potentially helping the magma rise and ripping it apart when released to the vacuum at the surface.

Geochemical data from MESSENGER have revealed details of Mercury’s surface composition, showing that it differs from the other rocky planets in the inner solar system. For example, the planet’s surface is enriched in S and C, and depleted in Fe, indicating that Mercury formed under much more reducing conditions than other planets. The surface is also enriched in Mg and depleted in Al and Ca. Observed elemental heterogeneities and percent levels of graphite suggest that Mercury underwent a magma ocean phase early in its history. These findings have important implications for understanding Mercury’s origin and evolution.

Extremely reducing conditions, such as those that prevailed during the accretion and differentiation of Mercury, change the “normal” pattern of behaviour of many chemical elements. Lithophile elements can become chalcophile, siderophile elements can become lithophile, and volatile elements can become refractory. In this context, unexpected elements, such as Si, are extracted to the core, while others (S, C) concentrate in the silicate portion of the planet, eventually leading to an exotic surface mineralogy. In this article, experimental, theoretical and cosmochemical arguments are applied to the understanding of how reducing conditions influenced Mercury, from the nature of its building blocks to the dynamics of its volcanism.